1. Field of the Invention
The present invention relates to a display drive apparatus, a display apparatus provided with the display drive apparatus, and a drive control method thereof, and more particularly, to a display drive apparatus which is applicable to a display panel formed by arranging a plurality of current control type optical elements which are driven by being supplied with a current corresponding to display data, a display apparatus provided with the display drive apparatus, and a drive control method thereof.
2. Description of the Related Art
In recent years, light weight and thin type display devices which consume a lower amount of electric power are conspicuously prevalent as monitors and displays of personal computers and video equipment. In particular, liquid crystal display (LCD) apparatuses are widely applied as display devices for mobile phones, digital cameras, personal data assistances (PDA's), and portable devices (mobile handsets) such as electronic dictionaries.
As a next-generation display device which follows such an LCD apparatus, research and development have been briskly made toward a full-scale popularization of a self-luminous type display device (a self-luminous type display) provided with a display panel in which organic electroluminescent elements (organic EL elements), inorganic electroluminescent elements (inorganic EL elements) or optical elements such as light emitting diodes (LEDs) are arranged in a matrix.
In particular, a self-luminous type display to which an active matrix drive mode is applied has a higher display response speed than that of the above-described liquid crystal display. Further, the self-luminous type display does not have viewing angle dependency, and can achieve an increase in luminance/contrast and in fineness of a display image quality. Furthermore, the self-luminous type display does not require a backlight as different from the liquid crystal display, and hence the self-luminous type display has very advantageous characteristics in the application to portable devices that a further reduction in a thickness and a weight and/or a further decrease in power consumption is possible.
Then, in such a self-luminous type display, various driving control mechanisms and/or control methods for controlling an operation of the optical elements have been proposed.
FIG. 35 is a schematic structural diagram showing a primary part of a conventional self-luminous display which is of a voltage control active matrix type.
FIG. 36 is an equivalent circuit diagram showing a structural example of a display pixel which is applicable to the conventional self-luminous type display.
Here, FIG. 35 shows a circuit configuration of a display pixel comprising a light emitting element formed of an organic EL element OEL as the optical element.
As shown in FIG. 35, a conventional organic EL display apparatus which is of an active matrix type generally has a configuration comprising: a display panel 110P in which a plurality of display pixels EMp are arranged in a matrix in the vicinity of intersections of a plurality of scanning lines (selection lines) SLp and a plurality of data lines (signal lines) DLp arranged to respectively extend in a row direction and a column direction; and a scanning driver 120P which is connected with the scanning lines SLp; and a data driver 130P which is connected with the data lines DL.
As shown in FIG. 36, each of the display pixels EMp comprises a pixel drive circuit DCp. The circuit DCp includes a thin film transistor (TFT) Tr111 having a gate terminal connected with the scanning line SLp and source and drain terminals respectively connected with the data line DL and a contact point N111, and a thin film transistor Tr112 having a gate terminal connected with the contact point N111 and a source terminal receiving a predetermined power source voltage Vdd. An organic EL element OEL has an anode terminal connected with a drain terminal D of the thin film transistor Tr112 of the pixel drive circuit DCp and a cathode terminal receiving a ground potential Vgnd lower than the power supply voltage Vdd. In FIG. 36, reference numeral Cp denotes a capacitor formed between the gate and the source terminals of the thin film transistor Tr112.
In the display apparatus comprising the display panel 110P constituted by the display pixels EMp having such a configuration, first, an ON-level scanning signal voltage Ssel is sequentially applied to the scanning line SLp in each row from the scanning driver 120P to turn on the thin film transistor Tr111 of the display pixel EMp (the drive circuit DCp) in each row, thereby setting the display pixel EMp in a selection state.
In synchronization with this selection timing, a gradation voltage Vpix having a voltage value corresponding to display data is generated by the data driver 130P and applied to the data line DLp in each column, and the gradation voltage Vpix is thereby applied to the contact point N111 (that is, the gate terminal of the thin film transistor Tr112) through the thin film transistor Tr111 of each display pixel EMp (the drive circuit DCp).
As a result, the thin film transistor Tr112 is turned on in a conductive state (i.e., a conductive state corresponding to the gradation voltage Vpix) corresponding to the potential (in a precise sense, a potential difference between the gate and the source) of the contact point N111. Thus, a predetermined driving current flows to the ground voltage Vgnd from the power source voltage Vdd through the thin film transistor Tr112 and the organic EL element OEL. Consequently, the organic EL element OEL operates to emit light with a luminance gradation corresponding to display data (the gradation voltage Vpix).
Subsequently, an off-level scanning signal voltage Ssel is applied to the scanning line SLp from the scanning driver 120P. Thus, the thin film transistor Tr111 of the display pixel EMp in each row is turned off, the display pixel EMp is set to a non-selective state, and the data line DLp and the drive circuit DCp are electrically disconnected. At this time, the thin film transistor Tr112 maintains an ON state in such a manner that a predetermined voltage is applied between the gate and the source terminals of the thin film transistor Tr112 based on a potential which has been applied to the gate terminal (the contact point N111) and held in the capacitor Cp.
Therefore, in the same manner as the light emitting operation in the selective state, a predetermined driving current flows to the organic EL element OEL through the thin film transistor Tr112 from the power supply voltage Vdd, thereby maintaining the light emitting operation. This light emitting operation is controlled to continue for, e.g., one frame period until the gradation voltage Vpix corresponding to the next display data is applied to (written in) the display pixel EMP in each row.
Such a drive control method is referred to as a voltage gradation specification mode (or a voltage gradation specification drive) since a current value of a driving current which flows to the organic EL element OEL is controlled to perform a light emitting operation with a predetermined luminance gradation by adjusting a voltage value of the gradation voltage Vpix applied to each display pixel EMp (specifically, the gate terminal of the thin film transistor Tr112 of the drive circuit DCp).
In the drive circuit DCp as shown in FIG. 36, the current path is connected in series to the organic EL element OEL, so that the element characteristics (particularly, the threshold voltage characteristics) of the thin film transistor Trr112 for drive which allows the flow of a driving current corresponding to the display data (gradation voltage) may change (shift) depending on the usage time, the drive history and the like. In such a case, a relation between a gate voltage (a potential of the contact point 111) and a driving current (a current between the source and the drain terminals) which flows between the source and the drain terminals changes, and thereby a current value of a driving current which flows at a predetermined gate voltage fluctuates (for example, decreases). As a result, it becomes difficult to stably realize a light emitting operation for a long period with an appropriate luminance gradation corresponding to the display data.
Furthermore, in the case where variation occurs in element characteristics (the threshold voltage) of the thin film transistors Tr111 and Tr112 in the display panel 110P for each display pixel EMp (the drive circuit DCp), or in the case where variation occurs in the element characteristics of the transistors Tr111 and Tr112 for each display panel 110P depending on the manufacture lots, the current value of the driving current largely varies for each display pixel or each display panel in the drive circuit which is of a voltage gradation specification mode, so that an appropriate gradation control becomes unable to be performed.